Flight paths are generally calculated in three dimensions, i.e., altitude and lateral position. To calculate a flight path in four dimensions requires the three-dimensional position of the aircraft to be specified over a number of points in time.
The ability to fly an aircraft according to a predetermined flight path with accuracy such that its position as a function of time is predictable is becoming increasingly important in air traffic control. This would allow air traffic control to relax separations between aircraft, leading to more efficient use of air space.
Although applicable to all phases of aircraft flight, one area that could particularly benefit from an enhanced ability to fly a four-dimensional flight path is in aircraft flying continuous descent approaches into airports. Typically, aircraft will approach an airport under the guidance of air traffic controllers. The air traffic controllers are tasked with ensuring the safe arrival of aircraft at their destination, while also ensuring the capacity of the airport is maximised. The former requirement is generally met by ensuring minimum specified separations are maintained between aircraft. Air traffic control is subject to uncertainties that may act to erode the separation between aircraft such as variable winds, both in speed and direction, and different piloting practices. Nonetheless, large numbers of aircraft can operate safely confined in a relatively small space since air traffic control can correct for these uncertainties at a tactical level using radar vectoring, velocity change and/or altitude change. As a result, a typical approach to an airport will involve a stepped approach where the aircraft is cleared to descend in steps to successively lower altitudes as other air traffic allows.
Air traffic noise around airports has important social, political and economic consequences for airport authorities, airlines and communities. An affordable way to tackle the noise problem in the vicinity of airports is to develop new guidance procedures that reduce the number of aircraft that fly over sensitive areas at low altitude with high thrust settings and/or with non-clean aerodynamic configurations (e.g. with landing gear and/or flaps deployed). Unfortunately, conventional step-down approaches act to exacerbate this problem as aircraft are held at low altitudes, where engine thrust must be sufficient to maintain level flight.
Continuous descent approaches (CDAs) are well known. These approaches see the aircraft approach an airport by descending continuously with the engines set to idle or close to idle. Clearly, continuous descent approaches are highly beneficial in terms of noise reduction as they ensure that aircraft are kept as high as possible above sensitive areas while at the same time reducing the noise production at the source through optimum use of the engine and flaps. Continuous descent approaches also benefit fuel efficiency, emission of pollutants and reduce flight time.
However, continuous descent approaches must be planned in detail before commencing the approach and cannot be subjected to tactical corrections to ensure safe aircraft separation like those used in conventional step-down approaches. To date this has obliged air traffic controllers to impose large spacings between aircraft to guarantee that the aircraft arrive at the airport separated by a safe distance, bearing in mind the potential for reduction in aircraft spacing as approaches are flown due to a result of wind changes and other uncertainties. Such an increase in separation results in an undesirable reduction in airport capacity.
The capacity penalty associated with continuous descent approaches has prevented their widespread use in airports and, to date, continuous descent approaches have mostly been used at airports with low levels of air traffic or at busier airports during quiet times (e.g. at night). Thus, it is desirable to be able to fly continuous descent approaches that minimise uncertainties in the four-dimensional position history of the aircraft. This would allow air traffic controllers to reduce safely the spacing between aircraft, thus satisfying the capacity needs of modern airports.